Micromechanical component

ABSTRACT

A micromechanical component includes a substrate and a cover layer deposited on the substrate, underneath the cover layer, a region of porous material being provided which mechanically supports and thermally insulates the cover layer. On the cover layer, a heating device is provided to heat the cover layer above the region; and above the region, a detector is provided to measure an electric property of a heated medium provided above the region on the cover layer.

This application is the national phase of PCT/DE02/02480 filed on Jul.6, 2002.

FIELD OF THE INVENTION

The present invention is directed to a micromechanical component havinga substrate and a cover layer deposited on the substrate, underneath thecover layer, a region of porous material being provided whichmechanically supports and thermally insulates the cover layer.

BACKGROUND INFORMATION

Although applicable to any number of micromechanical components andstructures, particularly sensors and actuators, the present invention,as well as its basic underlying problem definition are explained withreference to a micromechanical air-quality sensor which can bemanufactured using the technology of silicon surface micromechanics.

Existing air-quality sensors are implemented using a gas-sensitivematerial on a ceramic material. The gas-sensitive material changes itsresistance and/or its dielectric properties in dependence upon theconcentration of the gas to be detected. To obtain a good sensitivity,it is necessary to heat the gas-sensitive material. Thisdisadvantageously entails the use of a ceramic material and theassociated large type of design with respect to the substantial heatingpower to be expended and the long response time.

The method of etching silicon to make it porous (“anodizing”)constitutes related art, and it is described in numerous publications.The method of producing a cavity under a porous silicon layer islikewise already published (G. Lammel, P. Renaud, “Free-Standing Mobile3D Microstructures of Porous Silicon”, Proceedings of the 13^(th)European Conference on Solid-State Transducers, Eurosensors XIII, TheHague, 1999, 535-536).

SUMMARY OF THE INVENTION

An advantage of the micromechanical component according to the presentinvention is that it renders possible a simple and cost-effectivemanufacturing of a component having a thermally decoupled, heatablecover-layer area, upon which a detector is provided.

For example, the use of porous silicon makes it relatively simple toproduce a deep cavity having a superjacent cover layer. Moreover, it ispossible to make a defined region on a wafer porous up to a definedthickness, and, optionally, to oxidize to a higher valency in order tocreate a stable framework having low thermal conductivity.

In the exemplary implementation of an air-quality sensor using thismethod, one obtains the following further advantages:

-   -   low power consumption due to good thermal decoupling;    -   integration of a sensor element on the chip;    -   possible integration of a circuit on the sensor element;    -   very small size, along with any desired geometry of the porous        region;    -   low response time because of the small mass that has to be        retempered;    -   capacitive or resistive evaluation possible;    -   different materials are usable for the heating and/or measuring        resistors or electrodes;    -   a plurality of gas-sensitive materials may be employed on one        chip.

An idea underlying the present invention is to provide, on the coverlayer, a heating device to heat the cover layer above the region; and toprovide, above the region, a detector to measure an electric property ofa heated medium provided above the region on the cover layer.

In accordance with one preferred further refinement, the porous materialis formed from the substrate material. This is readily possible,particularly in the case of a silicon substrate.

In accordance with another preferred refinement, a hollow space isformed underneath the region of porous material.

In accordance with yet another preferred refinement, the cover layer isformed by oxidizing the substrate surface and the surface of the porousregion. This eliminates the need for depositing an additional coverlayer.

In accordance with yet another preferred refinement, the region ofporous material is completely oxidized. An oxidation of this kind isreadily possible because of the porous structure, and it enhances thethermal insulating capability.

Yet another preferred refinement provides for the component to be anair-quality sensor, the medium being a gas-sensitive medium, and thedetector having a capacitance detector and/or a resistance detector.

Still another preferred refinement provides for the detector to haveprinted conductors arranged on the cover layer.

Yet another preferred refinement provides for the detector to haveprinted conductors arranged on the insulation layer.

In yet another preferred refinement, the heating device extends at leastpartially underneath the medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of an air-quality sensor in accordance with afirst specific embodiment of the present invention.

FIGS. 2-4 show manufacturing steps for manufacturing the air-qualitysensor according to FIG. 1.

FIGS. 5-6 show manufacturing steps for manufacturing an air-qualitysensor in accordance with a second specific embodiment of the presentinvention.

FIG. 7 shows a cross-sectional view of an air-quality sensor inaccordance with a third specific embodiment of the present invention.

FIGS. 8-9 show manufacturing steps for manufacturing an air-qualitysensor in accordance with a fourth specific embodiment of the presentinvention.

DETAILED DESCRIPTION

In the figures, components which are the same or functionally equivalentare denoted by the same reference numerals.

FIG. 1 is a plan view of an air-quality sensor in accordance with afirst specific embodiment of the present invention.

In FIG. 1, reference numeral 6 denotes contact surfaces or contact pads;10 a semiconductor substrate; 40 a cover layer situated on the surfaceof semiconductor substrate 10; and 300 the boundary of a region inwhich, underneath cover layer 40, a region 30 (compare, e.g., FIG. 3) ofporous material is provided which mechanically supports and thermallyinsulates cover layer 40. In the present case, the substrate material issilicon and the porous material is anodized (porously etched) silicon.

In addition, reference numeral 50 denotes an insulation layer providedabove cover layer 40; 70 a heating resistor between cover layer 40 andinsulation layer 50; 350 the boundary of a region in which insulationlayer 50 is removed from above cover layer 40; 200 an interdigitalcapacitor situated on cover layer 40; and 150 denotes a gas-sensitivematerial which covers the interdigital capacitors.

To operate the sensor structure shown in FIG. 1, gas-sensitive material150 is heated by heating resistors 70, and the capacitance ofinterdigital capacitors 200 is measured in a generally known manner. Thegas-sensitive material changes its dielectric properties in dependenceupon the concentration of the gas to be detected. In this manner, thegas quality or concentration is able to be determined.

FIGS. 2-4 show manufacturing steps for manufacturing the air-qualitysensor in accordance with FIG. 1.

In FIG. 2, in addition to the reference numerals already introduced, 15denotes a mask, such as a resist mask, and 100 denotes circuitcomponents of a sensor circuit that is not explained more closely.Substrate 10 shown in FIG. 2 is a silicon substrate.

According to FIG. 3, using the known method of porous etching, astructure is produced in which the substrate material is made porous ina certain region 30, and a hollow space 20 is subsequently formedunderneath porous region 30. Thus a part of porous region 30 is removed,so the result is the structure shown in FIG. 3.

To produce the structure shown in FIG. 4, following removal of mask 15,porous region 30 is sealed by depositing cover layer 40, made, forexample, of nitride, oxide, oxinitride, silicon carbide, or polysilicon.Another possibility for forming cover layer 40 provides for oxidizingthe substrate surface and the surface of porous region 30.

It is not essential for this airtight sealing of hollow space 20 tofollow the fabrication of hollow space 20, rather, it may also beaccomplished as one of the last process steps. The latter has theadvantage that, during processing, cover layer 40 does not bump out,which would lead to aberrations in a structuring process. The internalpressure that ultimately arises in hollow space 20 is dependent upon thepressure conditions prevailing during deposition or oxidation.

The measuring capacitors of interdigital capacitor 200, heatingresistors 70, and optional measuring resistors (not shown) are thenproduced on cover layer 40. Further functional layers may be depositedand patterned between cover layer 40 and the printed conductors ofheating resistors 70, i.e., above the printed conductors.

Above the measuring capacitors of interdigital capacitor 200, followingapplication of insulation layer 50 which protects the formed structurefrom environmental influences, gas-sensitive material 150 is applied,which changes its dielectric properties as a function of theconcentration of a gas to be recorded.

The specific embodiment at hand has a hollow space 20, having anenclosed vacuum underneath cover layer 40, and region 30, in order toensure a good thermal insulation with respect to substrate 10 whengas-sensitive material 150 is heated by heating resistors 70.

FIGS. 5-6 illustrate the manufacturing steps used to manufacture theair-quality sensor in accordance with a second specific embodiment ofthe present invention.

In the second specific embodiment shown with reference to FIGS. 5 and 6,no hollow space is formed underneath substrate region 30′ that has beenmade porous. Rather, following removal of mask 15, porous region 30′ isimmediately sealed by deposition of cover layer 40 or by the oxidation.

In this context, the oxidation (not shown) has the advantage that theoxide has a lower thermal conductivity than the silicon, making itpossible to ensure a better decoupling from substrate 10. As in thefirst specific embodiment, the printed conductors, etc., are produced oncover layer 40.

FIG. 7 is a cross-sectional view of an air-quality sensor in accordancewith a third specific embodiment of the present invention.

In the third specific embodiment shown in FIG. 7, heating resistors 70are provided on cover layer 40, and the measuring capacitors ofinterdigital capacitors 200′ are provided on insulation layer 50, thusnot directly on cover layer 40 as in the above exemplary embodiments.The advantage of this arrangement is that the heating structure may beplaced directly underneath gas-sensitive material 150.

FIGS. 8-9 depict manufacturing steps for manufacturing an air-qualitysensor in accordance with a fourth specific embodiment of the presentinvention.

In accordance with FIG. 8, a two-layer substrate 10′, 10″ is provided,in which an epitaxial layer 10″ is provided on a wafer substrate 10′.Evaluation circuit 100 is additionally insulated by a buried region 110.The benefit of such a design is that the formation of porous region 30,30′ on bottom wafer substrate 10′ may be stopped by properly dopingcomponents 10′, 10″.

Although the present invention is described above on the basis ofpreferred exemplary embodiments, it is not limited to them, and may bemodified in numerous ways.

In the above examples, the air-quality sensor according to the presentinvention has been presented in simple forms in order to elucidate itsbasic principles. Combinations of the examples and substantially morecomplicated refinements using the same basic principles are, of course,conceivable.

For example, instead of changing the dielectric properties, it is alsopossible to change the electric resistance of the medium, e.g., of thegas-sensitive medium, using appropriate measuring electrodes.

In addition, it is possible to selectively etch porous region 30, 30′subsequently to or in-between the above process steps. For this purpose,one or a plurality of openings may be produced in cover layer 40,through which a selectively acting etching medium, in a fluid or gaseousstate, is able to partially or completely dissolve out the porousregion. The openings may subsequently be sealed again, a vacuum beingpreferably enclosed in hollow space 20 in the process in order to ensurean optimal thermal decoupling between cover layer 40 and substrate 10.The openings may likewise be deliberately not closed. In this manner,the middle cover layer region having functional elements may be formedin such a way that it is only still joined by a few land features(resist lines) to the substrate outside of the cavity (e.g., connectionby only two land features in the form of a bridge).

Also possible is the additional integration of a temperature sensor onthe cover layer outside of the porous region in order to precisely setor regulate the desired temperature.

It is also possible to provide different media on the cover layer or theinsulation layer above the porous region which are sensitive to variousgases. This makes it possible to measure a plurality of gases using thesame sensor element.

In addition, it is possible to realize the porous region so that itcontinues right through to the bottom side of the substrate.

Finally, any micromechanical base materials may be used, and not onlythe silicon substrate cited exemplarily.

In addition, the electric leads (not shown in FIG. 7) to theinterdigital structures may be situated underneath an electricallyinsulating protective layer. Also, the electrical connection by contactvias (openings) in the insulation layer may be implemented by electricalleads which are situated in the same plane as heating resistors 70.

10; 10′, 10″ Si substrate 6 contact pads 40 cover layer 300 boundary ofporous region under 40 350 boundary region without insulation layer 70heating resistor 200, 200′ interdigital capacitor 150 gas-sensitivemedium 15 mask 100 evaluation circuit 110 buried layer 20 hollow space30, 30′ porous region 50 insulation layer

1. A micromechanical component comprising: a substrate; a cover layerdeposited on the substrate; a region of porous material situatedunderneath the cover layer, the region mechanically supporting andthermally insulating the cover layer; a heating device situated on thecover layer for heating the cover layer above the region; a detectorsituated above the region for measuring an electric property of a heatedmedium provided above the region on the cover layer; and a structureincluding a hollow space underneath the region.
 2. The micromechanicalcomponent according to claim 1, wherein the porous material of theregion is formed from a material of the substrate.
 3. Themicromechanical component according to claim 1, wherein the cover layeris formed by oxidizing a surface of the substrate and a surface of theregion of porous material.
 4. The micromechanical component according toclaim 1, wherein the region of porous material is completely oxidized.5. A micromechanical component comprising: a substrate; a cover layerdeposited on the substrate; a region of porous material situatedunderneath the cover layer, the region mechanically supporting andthermally insulating the cover layer; a heating device situated on thecover layer for heating the cover layer above the region; and a detectorsituated above the region for measuring an electric property of a heatedmedium provided above the region on the cover layer, wherein thecomponent is an air-quality sensor, the medium is a gas-sensitivemedium, and the detector includes at least one of a capacitance detectorand a resistance detector.
 6. The micromechanical component according toclaim 1, wherein the detector includes printed conductors situated onthe cover layer.
 7. The micromechanical component according to claim 1,further comprising an insulation layer, and wherein the detectorincludes printed conductors situated on the insulation layer.
 8. Themicromechanical component according to claim 1, wherein the heatingdevice extends at least partially underneath the medium.
 9. Themicromechanical component according to claim 1, wherein the component isan air-quality sensor, the medium is a gas-sensitive medium, and thedetector includes at least one of a capacitance detector and aresistance detector.
 10. The micromechanical component according toclaim 5, wherein the porous material of the region is formed from amaterial of the substrate.
 11. The micromechanical component accordingto claim 5, further comprising: a structure including a hollow spaceunderneath the region.
 12. The micromechanical component according toclaim 5, wherein the cover layer is formed by oxidizing a surface of thesubstrate and a surface of the region of porous material.
 13. Themicromechanical component according to claim 5, wherein the region ofporous material is completely oxidized.
 14. The micromechanicalcomponent according to claim 5, wherein the detector includes printedconductors situated on the cover layer.
 15. The micromechanicalcomponent according to claim 5, further comprising an insulation layer,and wherein the detector includes printed conductors situated on theinsulation layer.
 16. The micromechanical component according to claim5, wherein the heating device extends at least partially underneath themedium.